The invention described herein relates generally to materials which exhibit nonlinear absorptive properties as described in U.S. Pat. application Ser. No. 08/965,945, now U.S. Pat. No. 6,267,913, which is incorporated herein by reference. More particularly, the present invention relates to structural variants of those materials which have high two-photon or higher order absorptivities and which, due to absorption of multiple photons, undergo chemistry with high efficiency, including, but not limited to, the creation of Lewis acidic species, Lewis basic species, radical species and ionic species.
For years, the possible applications of using two-photon or higher-order absorption for a variety of applications including optical limiting, optical memory applications, microfabrication, and rational drug delivery have been considered. There are two key advantages of two-photon or higher-order induced processes relative to single-photon induced processes. 1) Whereas single-photon absorption scales linearly with the intensity of the incident radiation, two-photon absorption scales quadratically with incident intensity and higher-order absorptions will scale with yet higher powers of incident intensity. As a result, it is possible to perform multiphoton processes with three dimensional spatial resolution. 2) Because these processes involve as a first step the simultaneous absorption of two or more photons, the chromophore is excited with a number of photons whose total energy equals the energy of multi-photon absorption peak but where each photon is of insufficient energy to excite the molecule individually. Because the exciting light is not attenuated by single-photon absorption in this case, it is possible to excite molecules at a depth within a material that would not be possible via single-photon excitation by use of a beam that is focused to that depth in the material. These two advantages also apply to, for example, excitation within tissue or other biological materials. In multiphoton lithography or stereolithography, the nonlinear scaling of absorption with intensity can lead to the ability to write features below the diffraction limit of light and the ability to write features in three dimensions, which is also of interest for holography.
It was discovered in accordance with an earlier invention (as described in U.S. application Ser. No. 08/965,945, which is incorporated herein by reference) that molecules that have two or more electron donors, such as amino groups or alkoxy groups, connected to aromatic or heteroaromatic groups as part of a π-electron bridge exhibit unexpectedly and unusually high two-photon or higher-order absorptivities in comparison to, for example dyes, such as stilbene, diphenyl polyenes, phenylene vinylene oligomers and related molecules. In addition, it was found that the strength and position of the two-photon or higher-order absorption can be tuned and further enhanced by appropriate substitution of the π-electron bridge with accepting groups such as cyano. It was also discovered in accordance with the earlier invention that molecules that have two or more electron acceptors, such as formyl or dicyanomethylidene groups, connected to aromatic or heteroaromatic groups as part of a π-electron bridge exhibit unexpectedly and unusually high two-photon or higher-order absorptivities in comparison to, for example dyes, such as stilbene, diphenyl polyenes, phenylene vinylene oligomers and related molecules. The strength and position of the two-photon or higher-order absorption can likewise be tuned and further enhanced by appropriate substitution of the π-electron bridge with donating groups such as methoxy.
Realization of many of the possible applications of two-photon or higher-order absorption by dyes rests on the availability of chromophores with both large two-photon or higher-order absorption cross sections and structural motifs conducive to excited state chemical reactivity.
In 1931 Göppert-Mayer predicted molecular two-photon absorption, [Göppert-Mayer, M. Ann. Phys. 1931, 9, 273] and upon the invention of pulsed ruby lasers in 1960, experimental observation of two-photon absorption became reality. Multiphoton excitation has found application in biology and optical data storage, as well as in other applications. [Strickler, J. H.; Webb, W. W., Opt. Lett. 1991, 1780; Denk, W.; Strickler, J. H.; Webb, W. W., Science 1990, 248, 73; Yuste, R.; Denk, W., Nature (London) 1995, 375, 682; Williams, R. M.; Piston, D. W.; Webb, W. W., FASEB J. 1994, 8, 804; Xu, C.; Zipfel, W.; Shear, J. B.; Williams, R. M.; Webb, W. W., Proc. Natl. Acad. Sci. 1996, 93, 10763; Rentzepis, P. M.; Parthenopoulos, D. A., Science, 1989, 245, 843; Dvornikov, A. S.; Rentzepis, P. M., Advances in Chemistry Series 1994, 240, 161; Strickler, J. H.; Webb, W. W., Adv. Mat. 1993, 5, 479, U.S. Pat. Nos. 4,228,861, 4,238,840, 4,471,470, 4,333,165, 4,466,080 5,034,613 4,041,476, 4,078,229]. Although interest in multiphoton excitation has exploded, there is a paucity of two-photon absorbing dyes with adequately strong two-photon absorption in the correct spectral region for many applications. Further, there is a paucity of such chromophores that upon multiphoton excitation undergo predictable and efficient chemical reactions.
Chemistry induced by the linear absorption of electromagnetic radiation (single photon) has been proposed and exploited for polymerization initiation, photocrosslinking of polymers, holography, computer memory storage, microfabrication, medicine, and biochemistry among many other applications. Chemistry induced by linear absorption, however, allows spatial control largely limited to two dimensions (i.e., a surface). The invention described herein allows spatial control of photoinduced chemistry over three dimensions.